1. Field of the Invention
The present invention relates to a tone signal processing apparatus in an electronic musical instrument and, more particularly, to a tone signal processing apparatus for reading out a PCM waveform of a tone from a waveform memory included in a PCM sound source, and performing sampling point interpolation or digital filter processing of the PCM waveform.
2. Prior Art
An electronic musical instrument of this type generally has a waveform memory corresponding to a plurality of kinds of musical instruments. The waveform memory stores waveforms in units of octaves of each instrument, and automatic accompaniment waveforms for, e.g., a chord accompaniment, a rhythm accompaniment, and the like in the form of PCM signals.
In general, an electronic musical instrument has, e.g., 16 parallel tone generation channels, and performs a maximum of 16 kinds of tone generations through these channels in correspondence with simultaneous depression of a plurality of keys and an automatic accompaniment.
A standard sampling rate of a PCM signal in one channel is, e.g., 50 kHz. PCM signals for 16 channels are time-divisionally read out from the waveform memory. A sampling rate for one channel corresponds to 800 kHz. Readout data for one channel include waveform data corresponding to two adjacent sampling points. Time-divisional data for one channel is obtained by interpolating between these two sampling points by an interpolation coefficient determined by an interval of depressed keys.
Successive 16 time-divisional data at a sampling rate of 800 kHz obtained by sampling point interpolation are accumulated by a channel accumulator to be synthesized into one tone signal at a sampling rate of 50 kHz. The tone signal includes musical tone signals for all the 16 channels, and 16 tones having 16 different tone colors can be simultaneously produced on the basis of this tone signal.
The accumulated tone signal is supplied to a digital filter before it is D/A-converted. This filter processing is performed to change tones to be produced to have brilliant or soft tone colors.
Note that Japanese patent laid-open application No. 23796/1989 discloses that the above-mentioned channel accumulator and the filter commonly use a single circuit.
In the tone signal processing apparatus, a filter used for adjusting a tone color uniformly processes an output from the channel accumulator. Therefore, different filter characteristics cannot be provided to waveform data of the respective channels.
For this reason, although after touch data common to all the depressed keys can be utilized as a filter coefficient, data such as a key velocity value corresponding to a depression pressure of each key, a key scaling value corresponding to an interval, and the like cannot be utilized as a filter coefficient.
For example, it is impossible to adjust tone colors in accordance with keyed notes (key scaling) in such a manner that brilliant tones are generated at a higher note, and soft tones are generated at a lower note. In addition, it is impossible to perform filter processing in units of channels in correspondence with parts of the channels in such a manner that brilliant tones are generated in melody channels, and bass tones are emphasized in chord or bass accompaniment channels.
In order to solve this problem, for example, the following circuit arrangement may be proposed. That is, filters having different filter coefficients are prepared in correspondence with the number of channels, and data of the respective channels obtained from the time-divisional data output from a sampling point interpolation circuit are supplied to the corresponding filters. However, in this method, filters having arrangements for directly executing formulas of the filters must be arranged in correspondence with the number of channels. For this reason, a circuit arrangement is considerably complicated, resulting in an increase in cost. Thus, such an arrangement is not practical for a simple electronic musical instrument.